This invention relates to a system and method for forming a contact material for a MEMS device.
Microelectromechanical systems are devices often having moveable components which are manufactured using lithographic fabrication processes developed for producing semiconductor electronic devices. Because the manufacturing processes are lithographic, MEMS devices may be made in very small sizes, and in large quantities. MEMS techniques have been used to manufacture a wide variety of sensors and actuators, such as accelerometers and electrostatic cantilevers.
MEMS techniques have also been used to manufacture electrical relays or switches of small size, generally using an electrostatic actuation means to activate the switch. MEMS devices often make use of silicon-on-insulator (SOI) wafers, which are a relatively thick silicon “handle” wafer with a thin silicon dioxide insulating layer, followed by a relatively thin silicon “device” layer. In the MEMS devices, a thin cantilevered beam of silicon may be etched into the silicon device layer, and a cavity is created adjacent to the thin beam, typically by etching the thin silicon dioxide layer below it to allow for the electrostatic deflection of the beam. Electrodes provided above or below the beam may provide the voltage potential which produces the attractive (or repulsive) force to the cantilevered beam, causing it to deflect within the cavity.
MEMS switches may fail if modest voltage is present across the open contacts when the switch is closed or opened. This is referred to as “Hot Switching”. This occurs because the contacts of a switch are microscopically rough. The true area of solid-solid interaction between the contacts is thus limited to that area at the tip of the tallest asperities. This true area of contact is typically much less than 1 um2. If voltage is present on the contacts during this brief time interval when the contact area is vanishingly small, immense heating of the asperity peaks that carry the instantaneous current spike can occur. This often exceeds the melting point of the contact materials. On the other hand, a very smooth surface may give rise to static friction (stiction) which may may it difficult or impossible to open the switch.
Previous attempts to improve reliability include chemical mechanical polishing (CMP) of materials. This will often contaminate the surface, which must remain atomically clean in order to provide low contact resistance. Roughness can also be reduced by tediously reducing the roughness of each of the layers in a tin film stack, such as the oxide/Ti/TiW/Au/Ru/RuO2 stack that is typically used in MEMS switches. Because these stacks are complex and multilayered, this is a time consuming and ad hoc process.
The contact resistance (CR) can increase or become variable with increasing open/close cycles, or the contacts can weld together, thus preventing the switch from opening. Generally speaking, the contacts tend to be unreliable. To combat this a vast range of contacting materials have been studied. These include mainly pure metals (Au, Mo, Ru, W, Ni, Pt, Pd), but alloys (AuPd, AuNi) and oxides (RuO2) are commonly found as well. Additionally, stacks of thin layers may be formed in various thickness ranges, where the various layers each provide a different function, such as adhesion, metal-metal diffusion barrier, thermal and electrical conductivity enhancement, and hardness. Constraints in the process and design often limit the effectiveness or extent of enhancement provided by these complex stacks of films. For instance, a thin oxide on the contacting surface of a Ru metal film can greatly improve the hardness at the contacts, and thus improve the reliability. However process methods to grow thin oxides on this metal are limited to 10-20 nm thickness. Over many millions of open/closure cycles, this extremely thin film can wear away completely.